In Multiple Input Multiple Output (MIMO) antenna technology multiple antennas are used in order to improve communication performance, e.g. in terms of increased data rates and link ranges. By estimating channel impulse responses and beamform signals based thereon, such improvements can be accomplished without requiring additional bandwidth or increased transmission power.
Massive MIMO is an emerging technique for wireless access wherein a very large number (e.g. hundreds) of phase-coherent antenna elements are used in a radio base station for serving a relatively small number (e.g. tens) of user terminals in a communication resource, e.g. the same time-frequency resource.
A fundamental assumption in massive MIMO is that the antenna array can acquire sufficiently accurate channel state information (CSI) to the user terminals, so that coherent (“closed loop”) beamforming can be applied in the downlink (link direction from the radio base station to the user terminal). It is thus assumed that the channel properties of the communication link can be acquired and there are various methods for this. For instance, for acquiring CSI for the uplink (link direction from the user terminal to the radio base station) the user terminals may be configured to transmit pilot signals, which the radio base station receives and estimates a channel response based on. Channel reciprocity may then be assumed for the downlink.
There are also various ways to perform beamforming in massive MIMO, which is briefly described next. Consider a radio base station with an antenna array with M antenna elements about to send beamformed data to K user terminals, where each user terminal comprises a single antenna. Let gk be a vector of size M that represents a channel response in a particular resource block from the antenna array to the kth user terminal. Then, at time t, the radio base station transmits in the downlink a linear combination of vectors as follows:Σk=1Kvksk(t)  (1)where {vk} are beamforming vectors associated with the K terminals, k=1, . . . , K, and {sk(t)} are symbols aimed at the respective K terminals at time instant t. The channel is considered constant within one resource block, and within each resource block a number T of symbols {sk(1); . . . ; sk(T)} can be sent to each terminal k. The beamforming vectors {vk} are chosen as functions of the (estimated) channel responses {gk}.
Operationally the beamforming method in Eq. (1) makes sure that power emitted by the radio base station antenna array is focused onto the geographical spots where it is known that the receiving user terminals are located. This assumes that the radio base station has had prior contact with the user terminals, and in particular, has estimated the channel responses {gk} to them, for each resource block. The term resource block here means the time-frequency space over which the channel is constant, often called “coherence interval” in the massive MIMO literature.
Before a link has been established with a user terminal, the radio base station does not know the channel response gk to the user terminal. This means that antenna array beamforming gain cannot be exploited for such user terminals. However, certain information, e.g. control information, must be sent on a periodic basis also to those user terminals to which the radio base station does not have channel state information. This kind of control information may comprise general system information that should simply be broadcast to everyone, and it may include “wake-up” (paging) messages intended to tell a particular user terminal that it should attempt to contact the radio base station on the uplink. In the present disclosure, this kind of information is denoted “control information” or “broadcast data”.
When CSI is unavailable, beamforming cannot be used and space-time block coding, which does not require CSI at the transmitter, may for instance be used. Space-time block coding is a way of efficiently exploiting spatial (transmit) diversity. For two transmitter antennas, the Alamouti scheme is often used, and for more than two antennas there are many well developed schemes, for instance various space-time or space frequency coding.
Space-time block coding exploits the spatial diversity by sending multiple copies of a data stream across the various antennas, and control information transmitted using space-time block coding will therefore consume and waste a significant amount of the scarce time-frequency resources.
Further, since beamforming cannot be used, the gains of using massive MIMO cannot be exploited for the transmission of the control information.